The smart window is a system that can sense and respond to external stimuli such as light, heat, or electricity. It controls light passage, promising the advantages of reversible control of indoor light and temperature in applications to next-generation household or industrial windows. Various systems have been tested over the years.
Intelligent liquids and hydrogels have resulted in the development of new materials that have found applications in many areas of optics and materials science. (See Y. D. Ling, M. G. Lu, Journal of Polymer Research 2009, 16, 29; G. Masci, C. Cametti, Journal of Physical Chemistry B 2009, 113, 11421; J. Zhang, L. Y. Chu, Y. K. Li, Y. M. Lee, Polymer 2007, 48, 1718; S. F. Yan, J. B. Yin, Y. Yu, K. Luo, X. S. Chen, Polymer International 2009, 58, 1246; J. R. Moon, J. H. Kim, Macromolecular Research 2008, 16, 489; J. P. Chen, T. H. Cheng, Polymer 2009, 50, 107; D. Chacon, Y. L. Hsieh, M. J. Kurth, J. M. Krochta, Polymer 2000, 41, 8257; B. Tasdelen, N. Kayaman-Apohan, O. Guven, B. M. Baysal, Polymers for Advanced Technologies 2004, 15, 528; D. Schmaljohann, D. Beyerlein, M. Nitschke, S. Zschoche, C. Werner, Abstracts of Papers of the American Chemical Society 2003, 225, U709; N. Morimoto, T. Ohki, K. Kurita, K. Akiyoshi, Macromolecular Rapid Communications 2008, 29, 672; Y. Y. Lang, S. M. Li, W. S. Pan, L. Y. Zheng, Journal of Drug Delivery Science and Technology 2006, 16, 65; H. Yang, Y. B. Tan, Y. X. Wang, Soft Matter 2009, 5, 3511.) These materials can be generally classified into three categories: electro-chromic, thermo-chromic and photo-chromic materials. For example, an electro-chromic window is usually composed of liquid crystal sandwiched between two panes of glass or plastic that are coated with indium tin oxide (ITO), a transparent conductive material. When an electrical voltage is applied to the ITO, the liquid crystal molecules change their orientations, so as to either allow or prevent the passing of light through the window.
Likewise, the thermo-chromic smart window typically uses a vanadium dioxide (VO2)-type material, which can undergo a structural transition from a semiconductor to a metal at a critical temperature Tc. This transition is accompanied by an abrupt change in the optical properties, from transparent to opaque.
These thermo-chromic materials are currently being considered for application in thermally adjustable smart windows, smart roofs, large-area displays for information and traffic engineering, and temperature-sensing applications in medical technologies. (See A. Seeboth, J. Kriwanek, R. Vetter, Journal of Materials Chemistry 1999, 9, 2277; W. Y. Chung, S. M. Lee, S. M. Koo, D. H. Suh, Journal of Applied Polymer Science 2004, 91, 890; K. C. Labropoulos, D. E. Niesz, S. C. Danforth, P. G. Kevrekidis, Carbohydr. Polym. 2002, 50, 393.)
The application of the so-called intelligent liquids and hydrogels is based on the opaque-transparent reversible property with temperature or electric fields, which can control the passage of light. In the last decade, reversible thermo-chromic liquids and hydrogels that are sensitive to external temperature have been extensively investigated, although thermo-chromic hydrogels besides poly(N-isopropylacrylamide) (PNIPAAm) hydrogel have rarely been described. PNIPAAm hydrogel is the best known temperature sensitive polymeric network, from which many kinds of hydrogels were derived. (See Y. D. Ling, et al.; G. Masci, et al.; J. Zhang, et al.; S. F. Yan, et al.; J. R. Moon, et al.; J. P. Chen, et al.; D. Chacon, et al.) All of these hydrogels present the properties of volume phase transitions when either temperature or pH changes. (See B. Tasdelen, et al.; D. Schmaljohann, et al.; N. Morimoto, et al.; Y. Y. Lang, et al.) Recently, Ling et al. reported one kind of PNIPAAm hydrogel prepared in brine, which undergoes transparent-opaque transitions with an increase in NaCl concentration. However, this kind of hydrogel with a high NaCl concentration breaks into pieces due to its poor strength and/or its specific volume phase transition. Yang et al. investigated one thermo-responsive supermolecular hydrogel consisting of cucurbit[6]uril and butan-1-aminium 4-methylbenzenesulfonate, which presents a gel(opaque)-sol(transparent) transition with a change in the temperature. Seeboth et al. and Chung et al. developed types of hydrogels with a dye embedded in a polyvinyl alcohol/borax/surfactant gel network, which respond to changes in temperature with reversible color changes.
In general, many of the currently known thermally-induced hydrogels have many advantages, such as being free of an organic solvent, being nonflammable, being degradable and having a high transparency, but the high cost of materials limits their practical applications, especially in the field of large-area smart windows and roofs. In contrast, the presently described stable, easily adjustable, innocuous, biodegradable liquid and hydrogel compositions can be made by using cheap and easily available industrial materials, such as thermally induced self assembly surfactants and hydrogel bases, such as agarose.